A nickel film in which there was a thickness gradient was grown in high vacuum (∼2 × 10-8 torr) on an (001) copper surface prepared inside the vacuum chamber. Examination of the specimen in an electron microscope, fitted with an X-ray microanalysis attachment, revealed that the nickel was strained to match the copper lattice until a thickness of 14.6 ± 3 A ̊ was reached. This result is in satisfactory agreement with theoretical predictions. At 14.6 ± 3 A ̊ misfit dislocations were formed, and their density increased as film thickness increased further. This increase was in fair agreement with theory until a film thickness of about 50 Å was reached. Thereafter, the elastic strain was significantly larger than the predicted value. It is suggested that this disagreement between experiment and theory was the result of work hardening of the nickel layer. © 1970.